1. Field of the Invention
The present invention relates to a metal halide lamp.
2. Related Background Art
Recently, metal halide lamps having an arc tube made of semitransparent polycrystalline alumina ceramic have been developed actively as substitutes for lamps having a quartz tube. Since this alumina ceramic tube has the heat-resistant temperature of 1,200xc2x0 C., which is higher than the heat-resistant temperature (1,000xc2x0 C.) of the quartz tube that has been used conventionally, a load imposed on the wall of the arc tube (hereafter called wall load) can be set higher, so that a metal halide lamp with a higher lamp efficiency can be obtained. With regard to this kind of lamp, low watt type lamps with a lamp input of 70 to 150W used for general interior lighting have been developed and commercialized mainly. However, high watt type lamps with a lamp input of 200 to 1,000W used for general exterior lighting also are being demanded now by the market.
An available low watt type metal halide lamp for interior lighting (e.g. for shops) having an alumina ceramic tube, for example in a case of a 150W type, has excellent properties of the lamp efficiency of 90 lm/W, the average color rendering index Ra of 90 and the rated life of 6,000 h. It should be noted that xe2x80x9cthe rated lifexe2x80x9d refers to an aging elapsed time when the luminous flux of the lamp is lowered to 70% of the value at the aging time of 100 h.
FIG. 8 is a cross-sectional view showing the construction of the arc tube in such a lamp. The arc tube 115 includes a light-emitting portion 116 made of polycrystalline alumina ceramic, which functions as a discharge arc region, and narrow tubes 117 and 118 provided at the both ends of the light-emitting portion 116. The light-emitting portion 116 and the narrow tubes 117 and 118 are integrated with each other by shrinkage fit. Inside of the light-emitting portion 116, a pair of tungsten electrodes 119 and 120 is provided. Into the narrow tubes 117 and 118, electrical supply members 121 and 122 made of niobium or an electrically conductive cermet are sealed hermetically with frit. At the discharge side ends of the electrical supply members 121 and 122, electrode rods extending from the tungsten electrodes 119 and 120 are connected. Within the arc tube 115, a light-emitting substance including metal halides such as DyI3, TmI3, HoI3, TlI, and NaI, Hg functioning as a buffer gas and a rare gas for supporting ignition such as Ar are each filled.
Basically, the shape of the arc tube in the above-stated low watt type metal halide lamp employing the alumina ceramic tube is the same as that of the conventional metal halide lamp having a quartz arc tube used for interior lighting. That is, the typical dimensions of the alumina ceramic arc tube with the configuration shown in FIG. 8, in a case of 150W type, for example, are the distance between electrodes Le of 10 mm and the inner diameter of tube xcfx86i of 10.6 mm. In this case, a so-called arc tube shape parameter Le/xcfx86i, which is a major parameter indicating the shape of the arc tube, becomes 0.94. The wall load xe2x80x9cwexe2x80x9d0 on the arc tube during operation of the lamp is 27 W/cm2. Note here that, assuming that the lamp watt and the internal surface area of the arc tube are W1a and Sa, respectively, then the wall load xe2x80x9cwexe2x80x9d can be represented by we=W1a/Sa.
On the other hand, as for the conventional lamp with a quartz arc tube, dimensions of the typical 150W type lamp are the distance between electrodes Le of 13.5 mm and the internal diameter of the tube xcfx86i of 13 mm, so that the value of Le/xcfx86i becomes 1.04. That is, the values of arc tube shape parameter Le/xcfx86i are set at almost the same level for both lamps. Therefore, it can be said that both of the arc tube in the conventional low watt type alumina ceramic lamp for interior lighting and the quartz arc tube metal halide lamp have a relatively thick and short shape.
As another example, JP10(1998)-144261 A discloses a so-called short arc type metal halide lamp of a 20 to 250 W type, employing an alumina ceramic tube. The feature of this lamp resides in that, as illustrated in FIG. 9, a discharge light-emitting portion in an arc tube 123 includes a cylindrical-shaped center portion 124 and hemispherical end portions 125 and 126. The value of arc tube shape parameter Le/xcfx86i of this lamp is specified within a range between 0.66 and 1.25, which corresponds to the low-watt type lamp for interior lighting shown in FIG. 8, whereas the wall load xe2x80x9cwexe2x80x9d is specified within a relatively high range of 25 to 35 W/cm2. In this way, this lamp can be grouped into a short arc type high-pressure discharge lamp for specialized lighting purpose, and the arc tube has a thick and short shape, which is the same as the above-stated low-watt type metal halide lamp for interior lighting. As for a light-emitting substance of this lamp, metal halides such as DyI3, TmI3, HoI3, TlI, and NaI as described above are filled therein.
Meanwhile, U.S. Pat. No. 5,973,453 discloses the shape of an arc tube in a high efficiency metal halide lamp for general exterior lighting, employing an alumina ceramic tube. In this lamp, a cerium halide based substance, whose emission spectrum lies in a wavelength region with a high spectral luminous efficiency, is filled as a light-emitting substance especially for realizing a lamp with a high luminous efficiency. As a specific light-emitting substance, cerium iodide (CeI3) and sodium iodide (NaI) are filled in a molar ratio of NaI/CeI3 ranging 3 from 25. Thereby, excellent properties of a high lamp efficiency of 130 lm/w and an average color rendering index Ra of 58 are realized in a 150 W type lamp. In this case, the value of arc tube shape parameter Le/xcfx86i is specified within a range greater than 5 in order to attain a high luminous efficiency and a long life required for general exterior lighting sources. As described later, such an arc tube has a thin and long shape, which is common to the conventional high-pressure sodium lamp and metal halide lamp for general exterior lighting. The wall load of the lamp is specified to be 30 W/cm2 or less.
Note here that the alumina ceramic tube was originally invented, developed, and adapted for a material of arc tubes for high-pressure sodium lamps for general exterior lighting. In this case also, the above-mentioned feature of the alumina-ceramic tube is exploited, so that, for instance in a 400 W type, a high luminous efficiency and long life high-pressure sodium lamp with a lamp efficiency of approximately 140 lm/W and a rated life of 2,000 h, and also with a relatively low average color rendering index Ra of 25, was developed and became widely available. Here, the arc tube of the high-pressure sodium lamp has a thin and long shape and the value of arc tube shape parameter Le/xcfx86i is increased with the increase in the lamp input. For example, the specific dimensions of the lamp of a 400 W type are the distance between electrodes Le of 84 mm and the inner diameter of arc tube xcfx86i of 7.65 mm, so that the value of Le/xcfx86i is set at 11.0. Whereas, those of the lamp of a 700 W type are 134 mm in Le and 9.7 mm in xcfx86I, so that the value of Le/xcfx86i is set at 13.0. The wall loads of the arc tube are set at approximately 15 W/cm2 in the 400 W type and 13 W/cm2 in the 700 W type.
In addition, also in the conventional quartz arc tube type metal halide lamp of a high watt type for exterior lighting, a relatively thin and long shaped arc tube is employed basically, as compared with the above-described thick and short shaped arc tube of a low watt type for interior lighting. In this case also, the value of arc tube shape parameter Le/xcfx86i is increased with the increase in the lamp input. For example, the typical values of Le/xcfx86i are set at 2.1, 2.2, 2.5, and 2.7 in a type of 300 W, 400 W, 700 W, and 1,000 W, respectively. In general, the rated life of the lamp is specified at 9,000 h or more.
As described above, the high-pressure discharge lamp can be classified into two types in terms of the shape of the arc tube. One is a so-called long arc lamp of a high watt type having a thin and long shape used for general exterior lighting. The other includes a low-watt type lamp for interior lighting such as for shops and a lamp for specific lighting purposes such as for projection, exposure and studio lighting. The latter lamps are so-called short-arc type lamps having a relatively thick and short shaped arc tube.
As for the former lamps, that is, conventional high-watt type high-pressure sodium lamps and metal halide lamps for general exterior lighting, the reason for employing a thin and long shaped arc tube is that these lamps need to have a long life property of 9,000 h or more in general, in addition to a high luminous efficiency as their lamp properties. That is to say, the life of the high-pressure discharge lamp mainly depends on the blackening of the arc tube, which is generated due to vaporization and scattering of the material constituting the electrodes at both ends of the arc tube. However, even when the lamp input is increased, a thinner and longer lamp shape allows prevention of the center portion of the arc tube from being influenced by the blackening due to the electrode constituting material, so that a lamp with a long life can be obtained. In addition, even when an alumina ceramic tube with superior durability and heat-resistance is employed, the value of the wall load xe2x80x9cwexe2x80x9d of the arc tube in the lamp for general exterior lighting is specified within a range of 23 W/cm2 or less in general. This range is equivalent for the wall temperature of approximately 1,150xc2x0 C. or less and is one of the conditions necessary to attain the above-mentioned long life of 9,000 h or more.
In order to respond to demands from the market, the inventors of the present invention have worked toward development of a 200 W or more of high-watt type metal halide lamp employing an alumina ceramic tube for general exterior lighting. Firstly, the inventors selected cerium iodide and sodium iodide as a light-emitting substance for obtaining a high lamp efficiency, which allows, for example, substitution of the conventional quartz arc tube metal halide lamp of a 400 W type, which is the leading mainstream of the lamps, with a lamp of a 300 W type.
However, when cerium iodide and sodium iodide are used as a light-emitting substance in a metal halide lamp with a thin and long shaped alumina ceramic tube, then problems specific to such a lamp of xe2x80x9ca crack in an alumina ceramic arc tubexe2x80x9d and xe2x80x9cdisappearance of the discharge arcxe2x80x9d occur, which are not generated in the conventional quartz arc tube metal halide lamp, and high pressure sodium lamp and low-watt type metal halide lamp that employ an alumina ceramic arc tube.
The above-described xe2x80x9ccrack in an alumina ceramic arc tubexe2x80x9d often occurs at a central portion of the tube when the arc tube is lit up in a horizontal position. Especially, an incidence of the crack is relatively higher during the initial aging time period of 60 minutes immediately after manufacturing the lamp. The crack is generated often along most of the tube diameter to extend across the whole tube, or a crack might be generated partially in an upper portion of the arc tube lit up in a horizontal position. Meanwhile, an incidence of the xe2x80x9cdisappearance of the discharge arcxe2x80x9d is higher within 30 to 300 seconds just after the starting of the initial aging time period immediately after manufacturing the lamp. It can be estimated that these two phenomena of xe2x80x9ca crack in an arc tubexe2x80x9d and xe2x80x9cdisappearance of the discharge arcxe2x80x9d depend on the cerium and sodium iodide (CeI3+NaI) based light-emitting substance itself, which is filled in the arc tube. These phenomena hardly occur in the lamp into which only NaI is filled, for example. In this way, it can be said that these phenomena are specific to the cerium and sodium iodide (CeI3+NaI) based light-emitting substance.
In addition, in order to respond to demands for a lamp with a higher luminous efficiency, a high efficiency metal such as cerium and praseodymium may be used, which results in a substantial increase in the load on the arc tube because of low vapor pressures of these metals. As a result, if the airtightness of the shrinkage fitting portion is not excellent, then the portion could not resist the vapor pressure during lighting, so that the lamp would burst.
In order to enhance the reliability of the shrinkage fitting portion, the thickness of the wall of that portion needs to be increased. However, when increasing the thickness of the wall of the shrinkage fitting portion, a thermal loss at the portion increases, which results in a decrease in the lamp efficiency.
Furthermore, the lamp has problems that temperatures at both internal ends of the arc tube are not uniform; the luminous efficiency decreases with the decrease in the amount of the light-emitting substance enclosed in the light-emitting portion because the light-emitting substance enters into the narrow tubes; and the resistance to pressure of the arc tube decreases.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a metal halide lamp with a high luminous efficiency and a long life, which is capable of securely preventing a crack in an arc tube and disappearance of the discharge arc from occurring.
It is another object of the present invention to provide a metal halide lamp with a high luminous efficiency and a long life, which has a sufficient resistance to pressure during lighting and is capable of making the temperatures at the both internal ends of the light-emitting portion uniform, decreasing thermal loss, and suppressing a decrease in the amount of the light-emitting substance that contributes to light emission.
It is still another object of the present invention to provide a metal halide lamp with a high luminous efficiency and a long life, which is capable of suppressing a fracture of the arc tube during the lifetime.
To achieve the above-stated objects, a metal halide lamp according to the present invention includes an arc tube made of light-transmissive ceramic, in which a pair of electrodes is provided and cerium iodide (CeI3) and sodium iodide (NaI) are enclosed as a light-emitting substance, wherein a molar composition rate NaI/CeI3 of the light-emitting substance is specified within a range of 3.8 to 10, inclusive, and a wall load on the arc tube ranges from 13 to 23 W/cm2, inclusive, and on a series of X, Y coordinates, where X denotes a value of a lamp watt (W) and Y denotes a value of Le/xcfx86i, where Le and xcfx86i denote a distance between the pair of electrodes and an internal diameter of the arc tube, respectively, values of the Le/xcfx86i and the lamp watt are specified to be within a range surrounded by lines passing through the points (200, 0.75), (300, 0.80), (400, 0.85), (700, 1.00), (1,000, 1.15), (1,000, 2.10), (700, 2.00), (400, 1.90), (300, 1.80), and (200, 1.70) in this stated order (more specifically, the diagonally shaded area in FIG. 4).
With this configuration, the degree of the curve of a narrowed discharge arc region, which is specific to a light-emitting substance including CeI3, can be mitigated and a localized increase in temperatures of the upper side of the arc tube can be lowered. Therefore, both of the problematic phenomena of a crack in the arc tube and disappearance of the discharge arc can be prevented. In addition, green spectrum radiation having a high relative luminous efficiency from CeI3 is increased, whereby a high lamp efficiency can be realized. Furthermore, temperatures of the wall of the arc tube can be kept within a range for suppressing sufficiently the reaction between the light-emitting substance and the alumina ceramic tube, and the blackening of the tube end portions can be mitigated, which can realize a metal halide lamp with a long life and a high luminous efficiency.
Another metal halide lamp according to the present invention includes an arc tube made of light-transmissive ceramic, the arc tube including: a light-emitting portion in which a pair of electrodes is provided and a light-emitting substance including at least one of cerium (Ce) and praseodymium (Pr) is enclosed; narrow tubes provided at both end portions of the light-emitting portion; and an electrical supply member that is sealed within one of the narrow tube and connected to one of the pair of electrodes. The light-emitting portion is configured so that a ratio of a minimum wall thickness to a maximum wall thickness thereof becomes 0.80 or more, and the light-emitting portion and each of the narrow tubes are integrated with each other.
With this configuration, a sufficient pressure resistance property can be realized during operation of the life, and a risk of the fracture in the arc tube can be lowered. In addition, there is no joint portion between the light-emitting portion and the narrow tube such as a shrinkage fitting portion. Therefore, superior airtightness can be realized and there is no need to form a partially thick wall portion. As a result, the thermal loss becomes small, which allows the vapor pressure of the light-emitting substance to be increased sufficiently, so that the lamp efficiency can be improved.
In the above-stated metal halide lamp, it is preferable that the both end portions of the light-emitting portion have a shape whose diameter becomes smaller gradually with increasing proximity to the narrow tube. Thereby, temperatures in the arc tube can be made uniform, so that the lamp efficiency can be improved.
In the above-stated metal halide lamp, the both end portions of the light-emitting portion may have a tapered shape.
In addition, in the above-stated metal halide lamp, a cross-sectional shape of the both end portions of the light-emitting portion may be formed in a curve.
In the above-stated metal halide lamp, it is preferable that the both end portions of the light-emitting portion have an approximately hemispherical shape. With these configurations, even when the lamp is operated in a state where the arc tube is disposed vertically, the light-emitting substance does not intrude into the narrow tube, and therefore a decrease in the amount of the light-emitting substance is suppressed. Therefore, the lamp efficiency can be improved.
Further, preferably, in the aforementioned metal halide lamp, protrusions or recesses may be formed on an inner wall of the both end portions of the light-emitting portion.